The term portal is a popular marketing label used to describe a large and very popular website. These large websites draw web users to them and then direct those same users to other websites. In technical terms, portals are no different from other websites. In commercial terms, they are usually focused on making money instead of serving public interest. Portals are occasionally websites that many people go to for information. Search engines such as Yahoo and Google; major news company websites such as CNN, Fox and CNBC; various commercial websites such as Apple's Quicktime Movie Trailer site; AOL's website, all serve as jump-off points to other places on the World Wide Web, usually places where people spend money.
This is why portals are such a big deal to marketing people. Piddly little websites that don't really link to places where people spend money (even if they do draw hundreds of thousands of visits per month) aren't of any interest to them. That's why InetDaemon has never been asked to place a banner on his site.